1. Scope of Invention
This invention relates generally to pressure control valves in hydraulic systems and more particularly to a pressure control/relief valve which eliminates pressure spikes and hydraulic shock in such systems.
2. Prior Art
Relief valves are used to provide a quick opening for excessive hydraulic pressure. They are characterized by a structure in which the regulated hydraulic pressure is applied directly to or through an orifice in a primary or relief piston. The relief piston area in a differential piston area-type relief valve is annular and measured in terms of the difference in areas resulting from the piston seating area and either the piston outside diameter or a defined smaller internal diameter. By this method, a large seating diameter is achieved, providing good flow capacity, and operating forces are reduced to provide high pressure capability in a reasonably small package. The force exerted on the relief piston annular area as a result of the hydraulic pressure tends to move the relief piston away from the source of the pressure, thereby unseating the valve to relieve the pressure. This opening pressure is typically opposed by a spring. Appropriate selection of the spring force and piston areas will provide a relief valve which will theoretically open at a desired pressure and close when the pressure falls below that pressure.
A pilot operated relief valve accomplishes a similar function as above in a hydraulic circuit. However, the inlet pressure causes fluid flow through an orifice in the relief spool or piston once a poppet or check valve is unseated, resulting in a differential pressure across the head of the relief piston or spool which acts to move the relief piston or spool to an open position.
My previous U.S. Pat. No. 4,742,846 discloses a differential piston relief valve which resolves many of the prior art shortcomings of hysteresis and instability due to the Bernoulli effect without sacrificing response time to open and close. However, the present invention, by introducing an additional differential piston, further provides automatic increases in set pressure while also serving to avoid hydraulic pressure spikes and shock.
All conventional relief valves such as the differential piston relief valve and the pilot operated relief valve are part of another shortcoming of conventional high pressure hydraulic control systems. When these hydraulic systems are actuated so as to energize a hydraulic actuator, cylinder or motor in conjunction with, for example, an overhead crane, pressure increases virtually instantaneously. As a result, there is a sharp hydraulic shock and/or spike in the pressurized fluid system which results in excessively abrupt energizing of the hydraulic motor. Not only is this operational limitation abusive to the system, but it may also be operationally detrimental in that the equipment being operated will exhibit too sharp a start-up.
The present invention provides a hydraulic buffer in that the biasing piston within the hydraulic pressure control valve will be displaced against a coil spring within the spring chamber of the valve producing linear, progressive change in spring force as hydraulic pressure increases to operating pressure. By this arrangement, hydraulic pressure rise rate is not only decreased from virtually instantaneously, but also reduces or eliminates any spike or overshoot in the system pressure.